An identical mark, called mappiq, has a different phonetic function, and can be applied to different consonants; the same mark is also employed in the vowel shuruk.

Dagesh and mappiq symbols are often omitted in writing. For instance, בּ is often written as ב. The use or omission of such marks is usually consistent throughout any given context. The two functions of dagesh are distinguished as either kal (light) or ḥazak (strong).

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A dagesh kal or dagesh qal (דגש קל, or דגש קשיין, also "dageshlene", "weak/light dagesh", opposed to "strong dot") may be placed inside the consonants בbet, גgimel, דdalet, כkaf, פpe and תtav. They each had two sounds: the original "hard" (plosive) sound, and a "soft" (fricative) sound. Before the Babylonian captivity, the soft sounds did not exist in Hebrew, but were added as a result of Aramaic-influenced pronunciation of Hebrew after this point in history. The letters take on their hard sounds when they have no vowel sound before them, and take their soft sounds when a vowel immediately precedes them, across word boundaries in Biblical Hebrew, but not in Modern Hebrew. When vowel diacritics are used, the hard sounds are indicated by a central dot called dagesh, while the soft sounds lack a dagesh. In Modern Hebrew, however, the dagesh only changes the pronunciation of בbet, כkaf, and פpe (traditional Ashkenazic pronunciation also varies the pronunciation of תtav, and some traditional Middle Eastern pronunciations carry alternate forms for דdalet).

* Only in Ashkenazi pronunciation Tav without a dagesh is pronounced [s], while in another traditions it is assumed to have been pronounced [θ] at the time niqqud was introduced. In Modern Hebrew, it is always pronounced [t].

** The letters gimmel (ג) and dalet (ד) may also contain a dagesh kal. This indicates an allophonic variation of the phonemes /ɡ/ and /d/, a variation which no longer exists in modern Hebrew pronunciation. The variations are believed to have been: גּ=[ɡ], ג=[ɣ], דּ=[d], ד=[ð]. The Hebrew spoken by the Jews of Yemen (Yemenite Hebrew) still preserves unique phonemes for these letters with and without a dagesh.[4]

*** The letter hey (ה) when word final is usually silent in order to indicate the presence of a word-final vowel. But when it receives a dagesh kal, the hey is pronounced instead of being silent. This is the rule in historic pronunciation, but in Modern Hebrew, this rule is generally ignored. However, when a non-silent word-final hey (הּ) occurs, it can take a furtive patach.

Dagesh ḥazak or dagesh ḥazaq (דגש חזק, "strong dot", i.e. "gemination dagesh", or דגש כפלן, also "dageshforte") may be placed in almost any letter, this indicated a gemination (doubling) of that consonant in the pronunciation of pre-modern Hebrew. This gemination is not adhered to in modern Hebrew and is only used in careful pronunciation, such as reading of scriptures in a synagogue service, recitations of biblical or traditional texts or on ceremonious occasions, and then only by very precise readers.

The presence of a dagesh ḥazak or consonant-doubling in a word may be entirely morphological, or, as is often the case, is a lengthening to compensate for a deleted consonant.
A dagesh ḥazak may be placed in letters for one of the following reasons:

The letter follows the definite article, the word "the". For example, שָׁמָיִםshamayim "heaven(s)" in Gen 1:8 is הַשָּׁמַיִםHashshamayim "the heaven(s)" in Gen 1:1. This is because the definite article was originally a stand-alone particle הַל hal, but at some early stage in ancient Hebrew it contracted into a prefix הַ 'ha-', and the loss of the ל 'l' was compensated for by doubling the following letter. In this situation where the following letter is a guttural, the vowel in 'ha-' becomes long to compensate for the inability to double the next letter - otherwise, this vowel is almost always short. This also happens in words taking the prefix לַ 'la-', since it is a prefix created by the contraction of לְ 'le-' + הַ 'ha-'. Occasionally, the letter following a He which is used to indicate a question may also receive a dagesh, e.g. Num 13:20הַשְּׁמֵנָה הִואHashshemena hi? - "whether it is fat".

The letter follows the prefix מִ 'mi-' where this prefix is an abbreviation for the word min, meaning "from". For example, the phrase "from your hand", if spelled as two words, would be מִן יָדֶךָmin yadekha. In Gen. 4:11, however, it occurs as one word: מִיָּדֶךָmiyyadekha. This prefix mostly replaces the usage of the particle מִן min in modern Hebrew.

The letter follows the prefix שֶׁ 'she-' in modern Hebrew, which is a prefixed contraction of the relative pronoun אֲשֶׁר asher, where the first letter is dropped and the last letter disappears and doubles the next letter. This prefix is rare in Biblical texts, but mostly replaces the use of אֲשֶׁר asher in Modern Hebrew.

It marks the doubling of a letter that is caused by a weak letter losing its vowel. In these situations, the weak letter disappears, and the following letter is doubled to compensate for it. For example, compare Ex. 6:7 לָקַחְתִּיlakachti with Num 23:28, where the first letter of the root ל has been elided: וַיִּקַּחvayyikkach. Lamed only behaves as a weak letter in this particular root word, but never anywhere else.

If the letter follows a vav consecutive imperfect (sometimes referred to as vav conversive, or vav ha'hipuch), which, in Biblical Hebrew, switches a verb between perfect and imperfect. For example, compare Judges 7:4 יֵלֵךְyelekh "let him go" with Deu. 31:1 וַיֵּלֶךvayyelekh "he went". A possible reason for this doubling is that the וַ 'va-' prefix could be the remains of an auxiliary verb הָוַיַ hawaya (the ancient form of the verb הָיָה hayah, "to be") being contracted into a prefix, losing the initial 'ha', and the final 'ya' syllable disappearing and doubling the next letter.

In some of the binyan verbal stems, where the Piel, Pual and Hitpa'el stems themselves cause doubling in the second root letter of a verb. For example:

In Masoretic manuscripts the opposite of a dagesh would be indicated by a rafe, a small line on top of the letter. This is no longer found in Hebrew, but may still sometimes be seen in Yiddish and Ladino.